The last version of Firefox for your Mac without you paying Steve Jobs more money is at ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/2.0.0.20/mac/en-US/Firefox%202.0.0.20.dmg
As for youtube and other sites, they check the [http://www.useragentstring.com/Firefox3.6.4_id_15947.php user agent string] sent by your browser, so you can change that to a higher Firefox version using the [https://addons.mozilla.org/en-US/firefox/addon/59 User Agent Switcher] extension.